Conventional alcohol-based heat transfer fluids, such as automobile antifreezes, have the tendency to corrode the various metal surfaces of the heat transfer or cooling system. A variety of metal surfaces are often involved including, for example, aluminum, copper, iron, brass, and solder. Aluminum corrosion can cause special problems.
In automobile coolant systems, any aluminum corrosion that occurs can not only damage the aluminum metal surface being attacked by corrosion, but may also adversely affect other parts of the coolant system due to a phenomenon called "transport deposition". By virtue of transport deposition, soluble aluminum corrosion products formed at engine surfaces may be carried to, and precipitated on, the cooler radiator tubes, where they form heat-insulating fins or coatings that impair heat transfer.
The general problems involved have been addressed by the industry over the years. The system involved is considered to be extremely complex. The innumerable solutions that have been proposed over the years dramatically underscore the complexities involved.
In addition to providing various approaches to solving the problems involved, the numerous prior patents and publications in this field reflect proposals for a seemingly endless list of organic and inorganic compounds as potential corrosion inhibitors. Thus, for example, various inorganic phosphates, silicates, phosphonates, borates, azoles, siliconates, nitrates, nitrites, and the like have been proposed in a wide variety of compositions.
What is considered to be the state-of-the-art technology is based upon what has been termed "silicone-silicate" chemistry. This emanates from the technology disclosed in U.S. Pat. Nos. 3,337,496 and 3,341,469. Numerous variations utilizing this basic technology have been utilized over the years.
This type of system has generally been perceived to have proven exceedingly effective over the years, providing a highly desirable antifreeze composition. However, there are several factors which have created a need for a new type of system.
First of all, the underlying chemistry in this type of system is extremely complex, and is not that well understood or fully under control. Such systems, when not fully under control, can allow polymerization of the silicate leading to the formation of gels in the cooling system. This formation of gels depletes active silicate from the antifreeze, thereby decreasing corrosion protection of the cooling system metal surfaces. Furthermore, modifications to such vehicular cooling systems are being required, fostered by the changing metallurgy in favor of substantially increasing usage of aluminum for various parts of the cooling system. However, these systems often also include conventional ferrous components; and therefore, the inhibitor must provide adequate protection against all types of cooling system metals.
In addition, many automobiles are being manufactured with smaller, harder working engines having higher operating engine temperatures than was the case in the past. Since metal corrosion activity increases at higher engine temperatures, the need for greater protection against corrosion also increases under these more severe operating conditions.
All of these factors have created the desire in some segments of the industry to utilize an antifreeze based on more effective corrosion-inhibitor systems. There is a continuing need for antifreezes and antifreeze concentrates that are physically stable prior to use, thereby facilitating ease of handling and not requiring special mixing by the customer or user, yet which provide adequate corrosion protection for the metallurgy being employed in fabricating vehicular cooling systems.
Certain organophosphate esters have been disclosed for use as lubricants and corrosion inhibitors. For example, ethylene oxide-containing organophosphates and propylene oxide-containing organophosphates and a method of preparing them are disclosed in U.S. Pat. No. 4,360,474.
As an additional illustration, German patent application 2,756,747 discloses PO/EO and PO/BO-containing phosphate esters (wherein "EO" denotes ethylene oxide, "EO" denotes propylene oxide, and "BO" denotes butylene oxide) useful as lubricants and corrosion inhibitors. The compounds of this publication are made by reacting polyphosphoric acid with aliphatic diols, triols, or tetrols. The organophosphates of the German application have molecular weights between 200 and 8,000. This German application is primarily directed to low-foaming metalworking lubricating fluids and does not disclose antifreeze formulations.
European patent application publication No. 59,461 relates to hydraulic fluids containing polyoxyethylene phosphate esters and salts thereof together with a nonionic oxyalkylene block copolymer, said copolymer having a molecular weight from about 950 to about 3,500 and having a polyoxypropylene content of 65 to 100 wt. percent based on the weight of the copolymer.